In general, the tunnels for subway have a possibility to cause deformation due to shocks propagated above the ground or vibrations and noises generated while the subway is operated. Thus, the tunnels are required for periodical measurement of convergence.
In the tunnels, the measurement of convergence is used to measure the behavior of an inner cross section of the tunnel to determine whether or not the tunnel is stable, and mainly includes the measurement of an inclination of the tunnel or gravitational acceleration caused by noises and vibrations.
Conventionally, the tunnel convergence is measured using an electric resistance gauge. However, this measurement has low precision, and is difficult to provide a reliable measurement value due to the influence of electromagnetic waves, particularly, while the subway is operated, so that it is not carried out in real time, and thus is performed after the operation of the subway is completed.
However, considering that the tunnel convergence is considerably subject to the physical influence of operation speed and load of the subway, the fact that the measurement cannot be performed while the subway is operated is regarded as a big problem.